1. Field of the Invention
This invention relates to improved surface analysis techniques wherein information concerning the surface composition is inferred from the decrease in a kinetic parameter, such as energy, associated with ions scattered off the surface.
2. Description of the Prior Art
Various techniques and apparatus for analyzing surfaces by scattering ions from the surfaces are disclosed in U.S. Pat. Nos. 3,480,774, issued to Smith on Nov. 25, 1969, 3,665,182, issued to Goff and Smith on May 23, 1972 and 3,665,185, issued to Goff on May 23, 1972. Such techniques include impinging a primary ion beam on a sample. The energy of the ions scattered at a given angle is thereafter measured and the intensity of a signal associated with the measured scattered ions is plotted as a function of the ratio of energy of the scattered ions to that of the impacting ions (i.e., E.sub.1 /E.sub.o) to at least semiquantitatively identify the elemental composition of the bombarded surface. In the techniques disclosed in these patents, signals resulting from the passage of sputtered ions through the energy analyzer have generally been observed and measured, even when neutralization of surface charge buildup is achieved. The presence of the signal generally attributed to sputtered ions decreases the overall signal-to-noise ratio, and obscures the detection of scattered ions having a low ratio of E.sub.1 /E.sub.o, which low ratios correspond to those ions that have lost a large fraction of their incident energy as a result of being scattered from surface elements of relatively low atomic mass.
The ion scattering spectrometers (ISS) disclosed in the patents referenced hereinabove appear to require the determination of the energy lost by a scattered ion. Assuming that the ions so measured are indeed scattered ions such that the mass thereof is also known, the mass of the target atom from which the primary ion is scattered at 90.degree. with respect to the incident primary ion beam may be inferred from the expression E.sub.1 /E.sub.o = (M.sub.2 -M.sub.1)/(M.sub.2 +M.sub.1), where E.sub.1 is the energy of the scattered primary ion, E.sub.o is the energy of the primary ion before scattering, M.sub.1 is the mass of the primary ion, and M.sub.2 is the mass of the target atom.
A similar relationship can be expressed in terms of the decrease in other kinetic parameters such as velocity and momentum associated with ions as a result of scattering. Thus a determination of the necessary information regarding the scattering events may be made by any of a large variety of analyzers, some of which are generally referred to as energy analyzers, while others may be more appropriately viewed as momentum analyzers (magnetic sector analyzers), or velocity analyzers (time-of-flight spectrometers), or velocity analyzers (time-of-flight spectrometers) but all of which determine some kinetic parameter (i.e., energy, momentum or velocity).
It should be noted that the calibration of most of the analyzers discussed hereinabove (with the exception of the time-of-flight analyzer) is dependent on the charge state of the ion being analyzed. In most cases, the ion is assumed to be singly charged. Where this is not the case, the actual mass or kinetic parameters of the ion can be determined by assuming the proper charge.
These analyzers are all limited in that they fail to distinguish between sputtered and scattered ions and must assume that the detected ions are indeed scattered ions, when in fact many of the detected ions are actually sputtered ions which contribute "noise" and background in the resultant spectra.